MOSCOW (Reuters) - Russia's Federal Security Service (FSB) has arrested 10 people from Central Asia with links to Islamic State who planned to carry out attacks with firearms and explosives in Moscow and St Petersburg, Russian news agencies reported on Saturday.

The FSB said in a statement that the 10 had been detained with the help of Tajikistan and Kyrgyzstan. It said they had been plotting assaults with automatic weapons and home-made explosives in public places in the two Russian cities.

Moscow last year launched a campaign of air strikes in Syria to help President Bashar al-Assad take on various rebel groups as well as the Islamic State militant group. The Kremlin has said its main objective is to crush Islamic State.

Thousands of Russians have been fighting in Syria with anti-Assad forces, Russian Prime Minister Dmitry Medvedev said in early November, warning of possible attacks by them when they return home.

The Ferghana Valley, a fertile and densely populated strip of land that straddles Uzbekistan, Tajikistan and Kyrgyzstan, is considered to be the heart of Islamist militancy in Central Asia.

Tajikistan also borders Afghanistan and is seen by some as a possible conduit for militant Islamists.

(Reporting by Polina Devitt; Editing by Andrew Osborn and Helen Popper)